Re: water running down a window

This is the method I would probably use:

Create a comp with an animated drop gliding from top to bottom with a slight trail behind it. I would do this with various masks or shape layers.

Duplicate this comp multiple times inside a new comp, offset them physically and temporally.

I would then create a gray image with various gradients littered about to use as a displacement map on that comp. This would give variation to the movement of your drops.